Frequency changer



Dec. 6, 1949 s, A, OTT 2,490,448

FREQUENCY CHANGER Filed Nov. 9; 1945 ld Q Hu lo f f, I D 8 QV) u O C Ikl 3 N g m.. k LS o QU) INVEN'I'OR. lg BYSTANLEYLOTT A TTORNEY PatentedDec. 6, 1949 FREQUENCY CHANGER y Stanley A. Lott, Sydney, New SouthWales, Aus'- tralia, assignor to Amalgamated Wireless (Australasia)Limited, Sydney, New South Wales, Australia, a company of New SouthWales Application November 9, 1945, Serial No. 627,751

(Cl. Z50- 36) 4 Claims.

This invention relates to modulators and like frequency changing systemsin which it is required greatly to attenuate one or both of the originalfrequencies.

In small beat frequency oscillators of prior art construction, it hasbeen common practice to employ multiple valves, (such as the well knownGAS), in conventional frequency converter circuits in which a low passfilter is provided in the output circuit for rejecting frequenciesexcept the required low frequency beats.

The satisfactory application of these prior art circuit arrangementsinvolves many design problems which are difficult to overcome in aneconomical manner.

In order to secure frequency stability it is usual, for reasons wellunderstood, to make the fixed and variable oscillators operate at as lowa frequency as possible. If, in addition, the upper limit of the beatfrequencies is required to be high, then the low pass filter in theoutput of the modulator or frequency changer' must be of elaboratedesign. For example, if the frequency of the fixed oscillator is 80 kc.and the frequency range of the variable oscillator 60-80 kc., in orderto provide output beat frequencies up to 20 kc., then the low pass ltermust transmit up to 20 kc. freely and then attenuate sharply at 60 kc..Furthermore, inductances in the lter will require to be rather large,firstly because the impedance level is high. Large coils, in addition tooccupying excessive space, tend to produce large fields which mayinteract upon the output.

From the foregoing it will be appreciated that the necessity for using afilter, in a circuit of the above mentioned type, for the purpose ofrejecting the undesired frequencies in the output circuit, complicatesdesign problems and adds greatly to the cost of the equipment.

In the past it has also been the practice, in order to suppress both ofthe input frequencies in the output, to employ various arrangements ofthe well known balanced modulator circuit.

These prior art modulator circuit arrangements require the use ofbalanced input and/or output transformers and eicient operation dependso n securing and maintaining a high degree of balance, a requirementthat is not readily realizable in practice.

In a copending application Serial No. 635,098, filed December 14, 1945,a modulator circuit arrangement has been suggested which securessatisfactory suppression of the modulating frequencies in the outputcircuit without the use of balancing transformers.

Brieflythe arrangement adopted in the above mentioned copendingapplication is as follows. Both modulating frequencies are appliedsimultaneously to the grid of one valve and to the cathode of a secondsimilar valve which has its con.

trol grid effectively earthed for all operating frequencies and has anunbypassed resistive impedance in its cathode circuit. Due to thisunbypassed impedance, so-called cathode phase inversion is achieved andthe modulating potentials are effectively applied to the grid of thissecond valve degrees out of phase with the modulating potentials appliedto the grid of the first valve.

The anodes of the two valves in this arrangement are connected -togetherand to an output circuit appropriately tuned to receive the desired beatfrequency.

Although the invention set out in the above mentioned copendingapplication overcomes most ofA the undesirable characteristics referredto above, it suffers from one serious disadvantage which prohibits itsuse in beat frequency oscillators, i. e., inability to prevent themodulating oscillators locking inf The object of the present inventionis to provide a comparatively simple modulator or frequency convertercircuit utilizing two valves in such a way that both of the modulatingfrequencies are greatly attenuated in the output, and in which thedisadvantages outlined above in respect to prior art practices aresubstantially avoided.

The above objective is achieved in accordance with the present inventionby an improved modulator arrangement comprising a pair of similarthermionic valves each having at least an anode, a cathode and a controlgrid, m'eans for applying modulating potentials to the control grid ofone valve, means for applying modulation potentials of a differentfrequency to the control grid of the other valve, a cathode circuitcommon to both valves and having a high value impedance connectedeffectively in series therewith, and an output circuit for obtainingenergy at a frequency equal to the sum and/or difference frequencies ofsaid modulation frequencies.

For a more complete understanding of the invention and the manner inwhich it is to be carried out, attention is now directed to thefollowing description in connection with the accompanying single figureof drawing, which illustrates one practical arrangement for performingthe invention. y

Referring to the drawing, valves VI and V2 employed in the presentexample are shown as a source 'l of modulating potential-of x'e'd ire-Aquency through a transformer A:Il "or other suit-v able coupling meansin conventional manner.

Modulating potentials from a separate source trol grid 4 of valve V2through a series resistor I and a shunt connectedresistance-capacitynet- Work 25, 25. 1

The screen grid of each yof 'valvesVl Aland V2 is bypassed for allsignal frequenciesby thecapacity Il and is supplied With .operatingapo-1.y

tentials from the positive terminal I2 of a suitable high tension supplysource (not shown) through the lead I3 an'd the series connectedresistor I4. The suppressorgrid of Ieach`valve is connected to itsassociated cathode. .Y

The cathodes k2 of the valves 'VI and V2 ar connected together and toearth I5 through a series connected network comprising an impedance unitI6 and the self biasing 'resistance capacity combination I'l-I8.

The nature of the impedance I6 should be such that it maintains a. highimpedance value for all frequencies to besuppressed.

In the present example the impedance I6 is constituted by a paralleltuned circuit L, C. The value of the bias1resistor I'l-is selectedtorprovide such biasing potentials on the grids 4 as will cause thevalves'VI and V2 to operateon a square law characteristic. It is vtobeunderstood that biasing potentials may be applied 4to :the-grids l inany known manner.

The anodes 3 of the valves `VI and V2 are joined together andconnectedto the ioutput terminal I 9 through theA coupling condenserE2l) and.

a filter network shown in a dotted rectangle. Operating potentials fromthe positive terminal l2 of the H. T. supply sourcefn'otrshown) aresupplied to the anodes 3 through the'anodefload resistor 22.

With the circuit arrangementthus described modulating potentials,applied'between` earth I5 and the grid 4 of valve VI, from the-'source1, will appear across the'cathodegload impedance in I5. As the grid -4of -the-valve V2 is connected to earth I5, the potentials appearingacross 'the cathode circuit will-vbe-appliedtofthe grid -4 of the valveV2 Witha phase4 'dier'encef of180.de= grecs as compared to' the-fpotentials applied'to the grid 4 of valve VI'.

The theory Yof cathode phase'inversion is so well known-that a detailed`510 9 of variable frequency are applied to -the conf Y.

explanation of the 'operation Vis fconsidere'd'unnecessary. In likemannermodulatingpotentials applied to the grid 4 "of valve V2,`from thesource 9, appear across the cathode loadimpedan'ce-'IG and are appliedto the gri'dli vofthe'valveVl `in trol grids of both valves, inphase=op'pos`ition,l without the'necessity of lusing push-.pull tra-ns#formers and in circumstanceswhichiprevent, to`

a argc extent, undesirable ylocking in of the modulating potentials.With the plates 3 connected togetherbthelcir- 1 cuit functions in known'manner-'tofp'r'oduce in the output circuit, "modulation@products-corresponding to the sum and difference frequencies of themodulating potentials applied to the grids 4 Whilst, at the same time,greatly attenuating the original modulating frequencies from the sources'I and 9.

In a practical embodiment of the invention as incorporated in'albeatfrequency oscillator, the fixed modulation-source "l was ladjusted tosupply an output frequency of 88 kc. whilst the variable frequencymodulation source 9 was adapted .to Yprovide-output oscillations over afrequency range of'from 68 to 88 kc. With this selection fof modulatingfrequencies, beat frequencies withinrthe range of from 0 to 20 kc. maybe obtained across-the output of the output filter and fed to anydesired utilization circuit either di- Y -rectlyorin controllableamplitude through a resistance potentiometer 22 or other convenientattenuator network in known manner.

The impedance unit I6 in the common cathode circuit rcomprlsesl aparallel resonant L, C circuit tuned, in the present example, to 78 kc.whichiinaintains a high impedance for all frequencies to be suppressed.

Thevalue of the resistor 25 and the condenser 2B 4in `the vshunt'connected resistance-capacity network inthe inputcircuit of the valveV2 is chosen to provide an .impedance which is low to harmonicsgenerated in the variable oscillator 9 buthighenough to supply suiicientgrid voltage at the fundamental frequency. In conjunction withalargeresistor at I5, this circuit also eliminates interaction due to voltagefrom the fixed oscillator 'I reacting with the variable oscillator 9 viathe grid capacity of valve V2.

The'circuit of the present invention permits thezoutput filter tube 'ofsimple construction, one section beingzsulicient tof'attenuate harmonicsand any fundamentalappearing in the output.

The outputlter inthe present example comprisesfa'sin'glepi sectionnetworkin which the impedance in theseries arm constituted by a paralleltuned circuit L1, C1, resonant at the same-frequencyas the tunedcircuitL, C in the Vcommon cathode lead, and the shunt arms arecapacities fasindicated at 23, 24 Ain the drawing.

'Ihe'i-rrvention could vbe applied to single sidebandor -suppressed'carrier modulation, in which casethe cathode load Would'betuned to the:carrierffrequency.

Although, inthe present example, the valves VI an`d=V2have been Ashownas pentodes it is to be clearly understoodthat performance of theinvention'is not restricted to the use of valves orthistypeiSimilar'itriodes or other convenient types may Vbe employed withoutaffecting the scope -of .the invention.

V4I claim:

1. In a modulationlsystem, twolelectron discharge devices each havingelectrodes including a-'control grid, a cathode and an anode, a firstsourc'eofoscillatory energy of carrier wave frequency connected betweenthe control grid and cathode of one device, a second source of energyv'of 'lower 4frequency connected between the control grid and cathodeofthe other device, airequencyfvariable impedance which is low atfrequencies 4materially higher than the frequency of the energy in saidlast named source coupling the 1fgrid of said last'named device to itscathode, -an impede-nce -e`fhich is high at the frequencies `ofsaidftvllol sources common to the connections between the control gridsand cathodes of both of said devices, whereby voltage variations on the-control grid of said other device -are set -up in 3. A system asrecited in claim 1, wherein said l0 Number last-named impedance includesa resistor in series with a parallel inductor and capacitor and acapacitor in parallel with said resistor.

4. A system as recited in claim 1, wherein said iirst-named impedancecomprises a resistor and capacitor in parallel.

STANLEY A. LOTT.

REFERENCES CITED The following references are of record in the i'lle ofthis patent:

UNITED STATES PATENTS Name Date 2,218,524 Chaiee Oct. 22, 1940 2,240,450Wolfskill Apr. 29, 1941 2,240,452 Wolfskill Apr. 29, 1941 2,349,933Mueller et a1. Feb. 12, 1946

